The present invention relates generally to implantable cardiac pacemakers, and more particularly to a pacemaker which is responsive or adaptive to patient exercise, as detected by movement or activity, to generate a pacing rate appropriate to the nature or type of the exercise.
Over the past several years various intrinsic and extrinsic parameters indicative of physical exercise by the pacemaker patient have been suggested for use in controlling pacemaker stimulation rate. The goal is to pace the patient's heart rate in a manner which is adaptive to the current condition of true exercise (or rest) of the patient, corresponding to the intrinsic heart rate of a healthy person with a normal functioning heart when experiencing the same conditions of exercise or rest.
For example, in German Patent No. DE 34 19 439 and related U.S. Pat. No. 4,688,573 (the "'573 patent"), the applicant herein discloses techniques for such rate responsive or rate adaptive pacing using the central venous blood temperature of the patient under various physiological conditions, with separate algorithms defining heart rate as a function of the blood temperature for states of rest and exercise. A decision rule is employed to select which of the algorithms is appropriate at any given time.
Physiological parameters such as blood temperature, blood oxygen saturation, and impedance attributable to respiration (minute ventilation), are exemplary of intrinsic parameters proposed in the past for pacemaker rate control, which, however, are relatively slow to respond to the onset of or changes in the level of exercise by the subject. Therefore, the pacemaker rate variations controlled by detection and use of such parameters tend to some extent to lag the events which make those variations necessary or desirable. Furthermore, detection of these parameters typically requires precision sensors which are expensive to manufacture and involve complex implantation procedures. Hence, although such sensors may quite accurately track the body's varying need for oxygen under differing conditions of rest and exercise for purposes of precise control of the pacing rate, they have not yet achieved great popularity among physicians and patients. Blood oxygen saturation sensing, for example, is considered to be a laboratory curiosity rather than a practical sensing technique for use in controlling pacing rate because of the relative complexity of the sensor apparatus and its implant procedure.
Faster response and less costly and complex manufacturing and implant procedures have been achieved through the use of activity or motion sensors such as accelerometers. It was suggested several years ago to convert mechanical forces, accelerations and pressures into electrical energy and/or signals for use in biomedical technology. One of the earliest techniques proposed in the patent literature was to generate electrical energy from piezoelectric crystals and other mechanoelectrical converters responsive to movement of the individual to power a device implanted in the individual, as disclosed for example in U.S. Pat. Nos. 3,659,615 and 3,456,134. In Journal Biomedizinische Technik 20, pp. 225-228 (1975), Funke described the use of a piezoelectric crystal embedded in silicone rubber and implanted in the pleural space between lung and ribs to detect respiratory rate, for controlling the pacing rate of the patient. U.S. Pat. No. 4,428,380 described using a piezoelectric sensor to measure cardiac activity.
Dahl may have been the first to disclose, in U.S. Pat. No. 4,140,132, the technique of detecting patient activity with a mechanoelectrical converter for the purpose of controlling the rate of a cardiac pacemaker. In Dahl's system, a weighted cantilever arm comprising a piezoelectric crystal was implanted in the patient, the patient's movements caused the cantilever arm to vibrate, the mechanical vibrations were converted to an electrical output signal by the crystal, and the output signal was used as a drive signal for the variable rate pulse generator of the pacemaker. Anderson described a similar system in U.S. Pat. No. 4,428,378 (the "'378 patent"), and used the amplitude of the high frequency content of the converter output signal which was purported to increase with patient movement, as a bandpass signal to control the stimulation rate in an activity-responsive cardiac pacemaker.
Devices such as activity or motion sensors have the distinct advantage that they provide virtually immediate response to patient movements or external forces to generate electrical signals for use in controlling the stimulation pulse rate of the implanted pacemaker. However, they have exhibited serious disadvantages, such as the adverse effect of noise disturbances external to the body, from nearby operating machinery, for example, or emanating from within the body, such as coughing, sneezing, laughing, or the like. Such disturbances are unrelated to physical exercise, but affected the heart rate when early accelerometer-type detectors were utilized for control of the pacemaker stimulation rate. The '378 patent and other prior art sources, such as Proceedings of the European Symposium on Cardiac Pacing, editorial Group, pp. 786 to 790, Madrid, 1985, and Biomedizinische Technik, 4, pp. 79 to 84, 1986, assumed that the maximum acceleration values detected by an activity-controlled cardiac pacemaker in a patient undergoing exercise occur in the range of the resonant frequency of the major body compartments such as the thorax and the abdomen, at approximately 10 Hz (hertz), and that the maximum sensitivity should be in the range above 10 Hz.
In U.S. Pat. No. 4,926,863 (the "'863 patent"), the applicant herein teaches that detection of the accelerometer or activity signal in a frequency range below 10 Hz, indeed below approximately 4 Hz, is actually highly indicative of true physical exercise by the patient. Moreover, restriction of detection signal frequencies to that range discriminates against and avoids undesirable response to disturbances external and internal to the body. As a result, the effect of disturbances unrelated to exercise can be significantly suppressed during use of a mechanoelectrical converter or like transducer to control the pacing rate.
The '863 patent observes that the amplitude maxima of activity-sensed signals arising from exercise such as walking, climbing stairs, running and bicycling occur with rhythmic motion of the body in the low-frequency range. In contrast, sudden spasmodic movements unrelated to true metabolic exercise produce amplitude maxima in the higher-frequency range, above approximately 10 Hz. Accordingly, the effects of the latter movements, as well as noise disturbances, can be excluded by limiting detection to only the low-frequency content.
By using the low frequency band and by establishing different baseline values as ongoing levels of comparison, the activity pacemaker disclosed in the '863 patent provides fast response and reliable pacing at a variable rate adapted to the level of physical exertion of the patient, closely corresponding to the heart rate of a normal healthy person under the same conditions of physical exertion.
In U.S. Pat. No. 5,031,615 (the "'615 patent"), which is a continuation of the '863 patent, the applicant herein discloses an accelerometer and related processing circuitry which are fabricated in hybrid semiconductor integrated circuit form. The accelerometer is designed in that form as a microminiature mechanoelectrical converter or transducer of suitably low power consumption which, as a consequence of its own construction or of use of associated filter circuitry, provides low pass filtering in a frequency band below about 4 Hz.
In U.S. Pat. No. 5,014,703 (the "'703 patent"), which is also a continuation of the '863 patent, the applicant herein discloses an activity pacemaker which detects patient movement, discriminates between detected movements related to true physical exercise and detected movements arising from forces or causes other than exercise by selectively limiting the detected activity signal to appreciable amplitude values in the low frequency range, samples and compares the detected movements related to exercise in successive equal intervals of time to determine whether the exercise is more vigorous or less vigorous than that which occurred during prior time intervals, and adjusts the pacing rate accordingly. Improved sensitivity to changes in workload is obtained by processing the low-pass accelerometer signal in successive intervals of each block of time, as a moving window.
In U.S. Pat. No. 5,031,614 (the "'614 patent"), the applicant herein discloses a simple and reliable technique for obtaining and using the values of both the frequency and the amplitude components of the activity (accelerometer) signal to control the pacing rate. Both components, frequency as well as amplitude, are obtained by examining only the amplitude of the processed activity signal, using the scanning or moving window technique. Comparison of difference values between blocks provides information regarding magnitude of the patient's exercise at any given instant, and frequency of the repetitions being undertaken in that exercise, whether it involves walking, running, bicycling or other activity. The technique has the advantages of fast reaction to changes in exercise and smoothing out inconsistent short term noise. The presence of both amplitude and frequency information regarding the nature of the patient's exercise allows control to be manifested by the smoothed output to more closely track the heart rate of a healthy person with a normal cardiovascular system engaged in similar activity, according to a particular selected characteristic curve or related data.
The '863, '615, '703 and '614 patents are incorporated herein in their entirety by reference.
These advances and refinements have served to make accelerometer-based cardiac pacing a simple and very effective device for tracking the patient's physical activity constituting true exercise and for controlling the pacing rate accordingly. It has been found, however, that a distinct nonlinearity of the accelerometer signals is exhibited, in the sense of differences in the signal, depending on the type of exercise in which the patient is engaged, notwithstanding identical workloads for the different activities. For example, even with both having the same workload, bicycle exercise generates a distinctly smaller accelerometer activity signal than is generated from treadmill (walking/-running) exercise. Use of the prior techniques of accelerometer-based cardiac pacing would lead to a considerably lower pacing rate for the former than for the latter. Yet, the patient's heart rate should be the same or at least closely similar in both instances.
It is a principal object of the present invention to provide apparatus and methods for controlling the stimulation rate of an activity pacemaker according to the specific type of exercise engaged in by the patient, so that the heart rate is adapted to the true workload.
Another significant object of the invention is to provide techniques for detecting and distinguishing between different types of physical exercise of the patient, and, in response, to perform variable cardiac pacing of the patient's heart according to the particular type of physical exercise which is discerned.